HTTPTransport 403 Forbidden with cloudflare and fourth-level subdomain

[alexeysofin]

Our on-premise sentry instance is located on fourth-level subdomain (sentry.dev.example.com) behind Cloudflare CDN with SSL. Some strange things happen when using default HTTPTransport with raven-python. Cloudflare CDN responds with 403 Forbidden to any request raven sends. At the same time sentry dashboard is working well and a chrome says that the certificate is valid. When switching to RequestsHTTPTransport, or moving to a third-level domain, everything starts working. Does anybody have an idea what is going on?

pip packages

attrs==19.1.0
backcall==0.1.0
boto3==1.9.111
botocore==1.12.115
certifi==2019.3.9
cffi==1.12.2
chardet==3.0.4
coreapi==2.3.3
coreschema==0.0.4
cryptography==2.6.1
decorator==4.4.0
Django==2.1.7
django-dirtyfields==1.3.1
django-environ==0.4.5
django-jet==1.0.8
djangorestframework==3.9.2
docutils==0.14
drf-yasg==1.14.0
gunicorn==19.9.0
idna==2.8
inflection==0.3.1
ipython==7.3.0
ipython-genutils==0.2.0
itypes==1.1.0
jedi==0.13.3
Jinja2==2.10
jmespath==0.9.4
jsonschema==3.0.1
MarkupSafe==1.1.1
parso==0.3.4
pexpect==4.6.0
pickleshare==0.7.5
prompt-toolkit==2.0.9
psycopg2==2.7.6.1
ptyprocess==0.6.0
pycparser==2.19
Pygments==2.3.1
PyJWT==1.7.1
pyrsistent==0.14.11
python-dateutil==2.8.0
python-json-logger==0.1.10
pytz==2018.9
PyYAML==5.1
raven==6.10.0
requests==2.21.0
ruamel.yaml==0.15.89
s3transfer==0.2.0
six==1.12.0
swagger-spec-validator==2.4.3
traitlets==4.3.2
uritemplate==3.0.0
urllib3==1.24.1
wcwidth==0.1.7
whitenoise==4.1.2

Python version: 3.7.2